1. Technical Field
The invention is related to video recorders and particularly to video cassette recorders.
2. Description of Related Art
Skip-field video recording and playback is a well-known technique for doubling the record time of a standard video tape cassette. Essentially, a skip-field video cassette recorder (VCR) records an incoming video signal while running the video tape at half-speed and recording only alternate video fields. Such devices are described in U.S. Pat. Nos. 3,524,018; 3,470,315; 3,470,316; 3,359,365, all to Kihara; Re. 29,999 to Hirota; and 3,678,186 to Narita. Generally, the prior art exemplified by these patents teaches various ways of angularly and axially positioning the magnetic heads on the rotary head drum of the video recorder.
U.S. Pat. Nos. 3,359,365 to Kihara and 3,678,186 to Narita teach skip-field recording with four magnetic heads instead of two. The latter Kihara patent teaches that the number of heads is doubled from two to four for purposes of recording the luminance and chrominance components through different heads. The Kihara patent also teaches an angular space between opposing heads related to the linear offset between adjacent recorded tracks on the video tape. The Narita patent requires a guard band between adjacent recorded tracks having a sufficient width so that the two pairs of heads trace recorded tracks and guard bands in alternate sequence to achieve skip-field recording and playback. Neither the Narita nor Kihara patents address zero guard-band recording.
U.S. Pat. No. Re. 29,999 to Hirota teaches a rotary head drum in a video recorder having three heads instead of two and further teaches axially offsetting two of the heads so as to twice reproduce each recorded field during skip-field playback. It also provides for delay of a half horizontal line to achieve the offset required for interlacing repeated fields. However, the Hirota patent does not address zero guard-band recording.
The idea of adding additional magnetic heads to the drum, or of selectively connecting various heads on the drum also has applications to VCR's which can playback while the tape is stopped (U.S. Pat. No. 4,396,954 to Sonoda et al.) and for permitting a selection of different tape speeds (U.S. Pat. No. 4,293,880 to Tsukada et al. and U.S. Pat. No. Re. 31,078 to Segerstrom), but none of these latter patents address the problem of skip-field recording.
Another technique for increasing the play time of a standard video tape cassette is to eliminate the guard band between adjacent recorded tracks on the video tape, in accordance with U.S. Pat. No. 4,293,880 to Tsukada et al. Elimination of the guard band tends to increase cross-talk between adjacent tracks during playback. This problem is solved, however, by differently orienting the gaps of two opposing heads on a video drum, as described in U.S. Pat. No. 4,426,666 to Kobayashi et al.
In summary, two alternate methods are known for increasing the record and playback time of a standard video tape cassette. One is the zero guard-band technique which eliminates the guard band between adjacent recorded tracks. The other is the skip-field recording technique which records only every other field.